JPH06220580A - Cold rolled sheet of high strength dual-phase steel having high and uniform n-value in wide strain range and its production - Google Patents

Abstract

PURPOSE:To provide a steel sheet having high and uniform n-value in a wide strain range and also having high strength and high ductility, to obtain a steel sheet free from defects and excellent in press formability by enabling the desirable production of the steel sheet, and to reduce the weight of a steel sheet for automotive body in particular. CONSTITUTION:This steel sheet is a high strength cold rolled dual-phase steel sheet satisfying TSXEl>=2500, having high and uniform n-value satisfying an operation expression f(n)=1/(n5+n20)<2>+25%¦¦n5-n20)¦¦<=4.5 in a strain range of 5-20%, and excellent in press formability (where n is an n-value at 5% strain n20 is an n-value at 20% strain and ¦¦n5-n20¦¦ is the absolute value of a difference between n5 and n20). By combinedly adding at least one element among Ti, Nb, V, Zr, and Mo and B and also optimizing continuous annealing conditions, the cold rolled sheet of high strength dual-phase steel having high and uniform n-value and excellent in press formability can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength composite cold-rolled steel sheet having a uniform and high n value in a wide strain range and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of global environment conservation, there has been a demand for improvement in fuel consumption of automobiles, and there is an active movement to reduce the weight of vehicle bodies for the purpose of achieving the improvement. That is, for this reason, the weight of the vehicle body is reduced by thinning the steel sheet used for automobiles, and the decrease in the vehicle body strength due to the thinning is compensated by the higher strength of the steel sheet. The demand for higher ductility is becoming more and more severe, and a material that has both high strength and high ductility is expected.

In order to meet such requirements, a steel having a TS (tensile strength) of 80 to 100 kgf / mm ² and an El (breaking elongation) of about 30% is obtained by utilizing the work-induced transformation of retained austenite. It is proposed in Japanese Laid-Open Patent Publication No. 60-43430. In these steels, local strain concentration does not occur due to work-induced transformation and the uniform deformability is greatly improved, but the workability after transformation of retained austenite to martensite is not so different from that of Dual Phase steel. Therefore, Japanese Patent Laid-Open Nos. 64-79321 and 1-2307
In Japanese Patent Application Laid-Open No. 15-27720 and Japanese Patent Application Laid-Open No. 1-272720, a steel sheet for improving local deformability such as stretch flangeability is proposed.

[0004]

However, in these steel sheets according to the prior art, the main object is to improve the absolute values of the uniform deformability and the local deformability, and the strain received during working is increased. No consideration is given to work hardening behavior (e.g. change in n value).

Since an actual pressed product is subjected to various strains depending on the location, the n value at each location is different even within the uniform strain range in the tensile test, resulting in non-uniform deformation. May cause defects. For example, 5%, which corresponds to the initial stage of processing
If the n value at a certain degree of strain is low, defects will occur early and the subsequent processing cannot be continued. Further, when the n value at a strain of about 5% is high but the n value decreases in the high strain region, defects are likely to occur at the shoulder portion of the die where the strain changes rapidly. Particularly in a steel sheet having a composite structure, the n value greatly changes as the strain increases, and thus defects due to such nonuniform deformation are likely to occur. Moreover,
This tendency becomes stronger as the strength of the steel sheet increases.

Therefore, for a high strength composite structure steel sheet,
Not only is the n value averagely high in a specific strain range, but the n value is required to be uniform in a wide strain range. In addition, Japanese Unexamined Patent Publication (Kokai) No. 2-217425 discloses a method for producing a steel sheet having an n value of 0.25 to 0.30 or more. The n value used therein is a strain of 10%. The average value in the range of 20% to 20% is not shown at all.

The present invention has been made to solve the above problems, and provides a high-strength composite microstructure cold-rolled steel sheet having a uniform and high n value in a wide strain range and a method for producing the same. The purpose is to provide.

[0008]

The present inventors produced various cold-rolled steel sheets containing retained austenite according to many steel types and annealing conditions, conducted tensile tests on these steel sheets, and examined the strain and n value. I investigated the relationship. An example of the result is shown in FIG. 1. The n value at each strain is
It was calculated by the formula of (n value) = (ε / σ) · (dσ / dε), which is defined by the true stress σ and the true strain ε during the tensile test.

Here, n value at 5% strain is n ₅ ,
When the n value at 20% strain is n ₂₀ , f (n)
= 1 / (n ₅ + n ₂₀ ) ² +25 × ‖n ₅ −n ₂₀ ‖ defined by the parameter f (n), the average size of the n value and the uniform distribution of the n value in a wide strain range. Sex can be evaluated at the same time. That is, it is clear from FIG. 1 that the steel sheet with f (n) of 4.5 or less has a uniform and high n value in a wide strain range.

The strain of 5% is selected by the present inventors as a typical strain amount in the initial stage of processing. There are various ways of selecting a typical strain amount when subjected to large processing, but if a too large strain amount is selected, it may exceed the range of uniform elongation.
Since the value cannot be calculated, 20% was judged to be appropriate.

To further describe FIG. 1, steel sheets having f (n) of 4.5 or less, such as steel sheet A and steel sheet C, have a uniform and high n value in a wide strain range. I understand. If f (n) exceeds 4.5, it cannot be said to have a uniform and high n value. That is, in FIG. 1, when n ₅ is remarkably high and n ₂₀ is considerably lowered as in the steel plate E, and when n ₅ is low and n ₂₀ is high as in the steel plate B, it means that the n value is high. Although it can be said that it does not have a uniform n value, there is a high possibility that partial non-uniform deformation will occur during press working. Further, when the difference between n ₅ and n ₂₀ is small like steel plate D or steel plate F but the value is low, it has a uniform n value, but it can be said that it has a high n value. Therefore, the pressing itself becomes difficult. The average n value (n _10-20 ) between strains of 10% and 20%
Is calculated by the formula ln (σ ₂ / σ ₁ ) / ln (ε ₂ / ε ₁ ), which is also shown in FIG. 1.
It is clear that _10-20 cannot be evaluated.

Also, the actual press workability and f (n) and
As an example of the relationship with TS × El, the overhang height due to overhang processing and f (n), TS × El (however, TS: 78-80kgf / mm ² )
The results of the investigation of the relationship are shown in FIG. The overhang height is
A steel plate having a blank diameter of 100 mm and a plate thickness of 0.7 mm was pressed against a perforated die having a diameter of 50 mm, and was pressed by a ball head punch having a radius of 23 mm to obtain the amount of pressing when a break occurred.
A bead is attached to the die to prevent it from flowing from the flange. According to FIG. 2, TS
It can be seen that a steel sheet having an xEl of 2500 or more and an f (n) of 4.5 or less has excellent overhanging property. On the other hand, TS × El
Is 2500 or more, or the overhanging height of the steel sheet with f (n) over 4.5 is not so large, and TS × El cannot evaluate the overhanging property.

As the TS level rises, the El level decreases and the overhang height decreases. However, in order to have the overhanging property matching each TS level, TS × El is 2500 or more. And f (n) is 4.5
There is no substitute for the following characteristics.

As described above, in order to obtain a steel sheet having high strength and excellent press workability, TS × El is high and f
It was found that low (n) is important, but the present inventors have further studied chemical components and annealing conditions for making TS × El 2,500 or more and f (n) 4.5 or less. It was As a result, the fine carbonitrides formed of Ti, Nb, V, Zr, and Mo and the solid solution B combine to fine-grain the structure of the steel sheet.
To keep the relationship between the cooling rate from the phase region and the holding temperature and holding time in the bainite transformation temperature region, f (n) should be 4.5.
I found it important to:

That is, the total addition amount of Ti, Nb, V, Zr, and Mo is X (wt%), the addition amount of B is B (wt%), the cooling rate is V _C (° C / sec), and the holding temperature is Is T _H (° C.) and the holding time is t _H (min), F (V _C , T _H , t _H , X,
_{B) = 1- (V C -20} ) / 60- (T H -430) 2 / 2500-
(T _H -5) ² /10-{500(X-0.008)} ^2- {500 (B-0.
003)} ² , where V _C ≧ 20, the function F (V
_{Setting C} , T _H , t _H , X, B) to 0 or more results in f
It has been found that (n) is necessary for producing a steel sheet having a value of 4.5 or less. 3 as an example, F in several steel plates having different C amount _{(V C, T H, t} H, X, B) and f
shows the relationship between the (n), C the amount of addition F be in the proper range _{(V C, T H, t} H, X, B) in the case is greater than or equal to zero f
Since (n) is 4.5 or less, a uniform and high n value can be obtained.

FIG. 4 shows the relationship between F (V _C , T _H , t _H , X, B) and TS × El in several kinds of steel plates with different C addition amounts. There is F
_{(V C, T H, t} H, X, B) the by zero or more TS
It can be seen that a steel sheet having an El of 2500 or more can be obtained. When the amount of C is 0.380%, F (V _C , T _H ,
Even if t _H , X, B) is less than 0, TS × El may be 2500 or more, but in this case, f (n) exceeds 4.5, as described in the explanation of FIG. It is as follows.

Therefore, the inventors of the present invention have made a detailed study on the addition amounts of C, Si, Mn, etc., and the temperature and time of heating and holding during annealing, and as a result, produced a steel sheet having both high strength and high ductility. The present invention has been accomplished by finding out the following, and is as follows.

TS × El ≧ 2500, and 5 to 20%
In the strain region of, f (n) given by the following formula is 4.5
A high-strength composite microstructure cold-rolled steel sheet having a uniform and high n value in a wide strain range, characterized in that:

[0019]

[Equation 3]

C: 0.1 to 0.3 wt%, Si: 1.0 to 2.2 wt%
%, Mn: 1.2 to 2.5 wt%, P: 0.02 wt% or less, S: 0.0.
01 wt% or less, B: 0.001 to 0.005 wt%,
Further, it contains 0.006 to 0.01 wt% in total of one or more elements selected from Ti, Nb, V, Zr and Mo,
When hot-rolling and cold-rolling a steel with the balance being Fe and inevitable impurities, and then performing continuous annealing, Ac ₁ +
Heat to a temperature of 50 ℃ or more and Ac ₃ or less for 30sec to 3min
After holding given by the following equation F (V _C, T _H,
cooling rate V _C (° C.) such that t _H , X, B) becomes 0 or more.
/ Sec), holding temperature T _H (° C.), holding time t _H (min), and holding of a high-strength composite microstructure cold-rolled steel sheet having a uniform and high n value in a wide strain range characterized by holding. Production method.

[0021]

[Equation 4]

[0022]

The operation relationship of the above-described one according to the present invention will be described. The n value at 5% strain is n ₅ ,
When the n value at 20% strain is n ₂₀ , f (n)
= 1 / is ^{_{(n 5 + n 20) 2}} + 25 × ‖n 5 -n 20 ‖ defined in the f (n), 4.5 the a steel sheet less, uniform in a wide strain range, yet high n value 1 is as described above and shown in FIG.

The reasons for limiting the content range of alloying elements in the manufacturing method of the present invention are as follows. C concentrates in the austenite during the bainite transformation of the supercooled austenite and stabilizes the austenite, thereby generating retained austenite. Although a high n value can be obtained by the work-induced transformation of this retained austenite, 0.1% or more of C is required to obtain the effect. On the other hand, when the content exceeds 0.3%, the amount of retained austenite increases, but a uniform n value cannot be obtained and f (n)
Since it exceeds 4.5, the upper limit of C is set to 0.3%.

Si is a ferrite stabilizing element, which is indispensable for obtaining a stable ferrite-austenite two-phase structure at the time of heating in continuous annealing, and the addition of 1.0% or more is necessary. However, excessive addition extremely raises the transformation point of the steel, so that the heating temperature must be raised during annealing, which increases the production cost. Therefore, the addition amount is limited to 2.2% or less.

Mn shifts the nose of the ferrite-pearlite transformation to the long side, and is an essential element for the formation of retained austenite by the bainite transformation. Moreover, like C, it is an austenite stabilizing element and is necessary for obtaining a high n value. These effects are not exhibited with addition of less than 1.2%, so the lower limit is 1.2%.
However, since the bainite transformation is also delayed, the holding time in the transformation temperature range must be lengthened, which lowers the productivity, so its upper limit is made 2.5%.

P is a ferrite stabilizing element and has the same effect as Si, but segregates at the grain boundaries to cause embrittlement and causes local defects during press working. Good. Therefore, in the present invention, it is limited to 0.02% or less.

[0027] S is desired to be as small as possible because it significantly reduces the ductility of steel, and is limited to 0.01% or less.

By adding one or more elements selected from Ti, Nb, V, Zr, and Mo, the fine carbonitride can be used to refine the structure, and the retained austenite can be uniformly distributed. By doing so, the work-induced transformation can be effectively caused. As a result, a uniform n value can be obtained in a wide strain range, but in order to obtain this effect, at least 0.006% or more in total must be added. However, when a large amount of carbonitride is precipitated by excessive addition, this carbonitride lowers the n value of the steel plate, and f
(n) exceeds 4.5. Therefore, the upper limit is 0.01%
And

B has a function of making the structure fine by being dissolved in steel. In order to make the structure sufficiently fine with only fine carbonitride, the addition amount of Ti, Nb, V, Zr, and Mo is 0.01%.
Although it is preferable that the amount is larger than that, the level of the n value is lowered in that case, and therefore it is necessary to combine the fine-graining effect of the solid solution B. In order to exert this effect, it is necessary to add at least 0.001% or more of B. However, when the content exceeds 0.005%, the level of the n-value still decreases, so the upper limit is 0.005%.

[0030] Incidentally, the above-mentioned Ti, Nb, V, Zr, Mo, the addition amount in the range of B, the cooling rate during the continuous annealing, the holding temperature, the function F (V _C including retention time, T _H, t _H , X, B), and even within the above range of addition amount, f
(n) may exceed 4.5.

The reason for the limitation in the production of the steel slab having the above-mentioned components is as follows.
The material is continuously annealed after cold rolling, and the heating temperature is Ac ₁ + 50 ° C. or higher and Ac ₃ or lower. When the heating temperature is lower than Ac ₁ + 50 ° C, the amount of austenite in the two-phase region is insufficient, so the amount of retained austenite finally produced does not reach a sufficient amount to exert the effect of work-induced transformation. . On the other hand, when it is higher than Ac ₃ , it is completely austenitized, so that it becomes impossible to generate residual austenite without causing the concentration of C and therefore the heating temperature is Ac ₁ + 50 ° C. or higher Ac Limited to ₃ or less.

If the holding time in the above temperature range is shorter than 30 sec, a homogeneous two-phase structure cannot be obtained, and if it is longer than 3 min, the structure becomes coarse. Or f (n) exceeds 4.5. Therefore, the heating and holding time is set to 30 sec to 3 min.

In the present invention, the total addition amount of Ti, Nb, V, Zr, and Mo is X (wt%), the addition amount of B is B (wt%),
When the cooling rate is V _C (° C./sec), the holding temperature is _TH (° C.), and the holding time is t _H (min), F (V _C , _TH , t _H ,
X, B) = 1- (V C -20) / 60- (T H -430) 2/25
00- (t _H -5) ² /10-{500(X-0.008)} ^2- {500
(B-0.003)} ² function defined by F (V _C, T _H,
It is very important to carry out cooling and holding after heating and holding so that t _H , X, B) becomes 0 or more. However, when the cooling rate is lower than 20 ° C./sec, a large amount of ferrite or pearlite is generated and TS × El becomes low, so V ≧ 20 ° C./sec. If the cooling rate is too fast, or if the holding temperature or holding time is not within the appropriate range, F
(V _C , T _H , t _H , X, B) is less than 0, but in such a case, the amount of retained austenite produced is small and at the same time, the retained austenite becomes unstable with respect to an external force. Therefore, in the low strain region, a high n value is exhibited due to the work-induced transformation, but the n value sharply decreases in the high strain region. Therefore, TS × El decreases,
f (n) exceeds 4.5.

[0034]

EXAMPLE A specific example of the present invention will be described below with reference to Examples 1 and 2.

[0035]

Example 1 The chemical compositions of representative steels according to the present invention and comparative examples which were specifically adopted by the present inventors are as shown in Table 1 below, and the steels b, c, f, g, and j, k, p,
q and u are examples of the present invention, and others are comparative examples.

[0036]

[Table 1]

Each of the steels having the composition shown in Table 1 was melted, cast, and hot-rolled and cold-rolled according to the usual method to obtain a cold-rolled sheet having a thickness of 0.7 mm. After the cold rolling thus obtained, after heating and holding at 800 ° C. × 60 sec, it is cooled at a cooling rate V _C : 45 ° C./sec, and a holding temperature T _H : 42.
Continuous annealing was performed at 5 ° C. for a holding time t _{H of} 5 min. The obtained product was subjected to a tensile test using a JIS13B test piece to quantify the volume ratio (γ _R ) of retained austenite. The results are shown in Table 2. That is, according to the results of Table 2, steels b, c, f, g, j, k according to the present invention,
TS × El of p, q and u is 2500 or more, and f (n) is 4.
Since it is 5 or less, high strength, uniform and high n
It has a value, and it can be seen that the press workability is excellent. On the other hand, steels a, e, h, i, l, which are comparative examples,
For m and n, TS × El is significantly reduced because the addition amounts of C, Si, Mn, P and S are not appropriate. For steel d, TS ×
Although El is high, f (n) exceeds 4.5. Further, steels o, r, t, and v have f (n) exceeding 4.5 because the total addition amount of Ti, Nb, V, Zr and Mo and the addition amount of B are excessive and deficient. It is clear that is low or not uniform. Further, the steel s contains B: 0.001 to 0.005 wt% and further contains one or more elements selected from Ti, Nb, V, Zr and Mo in a total amount of 0.006 to 0. Function F (V _C , T _H , t _H , X, B) although it contains 0.01 wt%
The value of is less than 0, and f (n) exceeds 4.5.

[0038]

[Table 2]

[0039]

Example 2 The cold-rolled steel sheet of Steel C in Table 1 was continuously annealed under the conditions shown in Table 3 below. This table 3
Where T _A is the heating and holding temperature, t _A is the heating and holding time,
V _C is the cooling rate, T _H is the holding temperature, and t _H is the holding time.

[0040]

[Table 3]

Table 4 shows the results of examining the mechanical properties and the residual austenite volume ratio (γ _R ) of the products obtained by the continuous annealing as shown in Table 3 above.

[0042]

[Table 4]

That is, sample Nos. 2 and 7, which are examples of the present invention,
8, 11, 12, 15, and 16 are properly annealed, and TS × El is 2,500 or more and f (n) is 4.5 or less, showing that the press workability is good. . On the other hand, sample Nos. 1 and 3 in which T _A is outside the range of the present invention and sample Nos. 4 and 5 in which t _A is outside the range of the present invention have low TS × El, and f ( n) is also over 4.5. Also,
In sample No. 6, since V _C was too small, TS × El significantly decreased, and further, sample Nos. 9, 10, 13, 14, 17, 1
In 89, F (V _C , T _H , t _H , X, B) is less than 0 and TS × El decreases, or TS × El is high but f (n)
It is clear from the fact that the value exceeds 4.5, it is highly possible that any of the samples has a defect during press working.

[0044]

According to the present invention described above, a high strength and high ductility steel sheet having a uniform and high n value in a wide strain range is provided, and its preferable production is made possible to cause defects. Since it is possible to perform press processing that is not necessary, it has a great industrial utility value, and in particular, it is an invention that has a great industrial effect because it is extremely useful for reducing the weight of automobile bodies.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between strain and n value.

FIG. 2 is a graph showing the relationship between overhang height and f (n) · TS × El.

FIG. 3 is a graph showing the relationship between F (V _C , T _H , t _H , X, B) and f (n).

FIG. 4 is a graph showing a relationship between F (V _C , T _H , t _H , X, B) and TS × El.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akihiko Nishimoto 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (2)

[Claims] 1. TS × El ≧ 2500, and 5 to 20.
In the strain region of%, f (n) given by the following equation is 4.
A high-strength composite microstructure cold-rolled steel sheet having a uniform and high n value in a wide strain range, which is 5 or less. [Equation 1] 2. C: 0.1 to 0.3 wt%, Si: 1.0 to 2.2 wt%
%, Mn: 1.2 to 2.5 wt%, P: 0.02 wt% or less, S: 0.0.
01 wt% or less, B: 0.001 to 0.005 wt%,
Further, it contains 0.006 to 0.01 wt% in total of one or more elements selected from Ti, Nb, V, Zr and Mo,
When hot-rolling and cold-rolling a steel with the balance being Fe and inevitable impurities, and then performing continuous annealing, Ac ₁ +
Heat to a temperature of 50 ℃ or more and Ac ₃ or less for 30sec to 3min
After holding given by the following equation F (V _C, T _H,
cooling rate V _C (° C.) such that t _H , X, B) becomes 0 or more.
/ Sec), holding temperature T _H (° C.), holding time t _H (min), and holding of a high-strength composite microstructure cold-rolled steel sheet having a uniform and high n value in a wide strain range characterized by holding. Production method. [Equation 2]